Process for combustion or decomposition of pollutants and equipment therefor

ABSTRACT

Process to ascertain combustion and/or decomposition of gaseous and/or by air or other gas carried particulate pollutants. The invention makes possible an effective and safe combustion and/or decomposition of pollutants not desired to escape into the environment. The pollutants are fed into a combustor (1) in which the pollutants are self combusted and/or are made to self decompose, whereby in the combustor outgoing gas is made to heat ingoing gas and/or by air or other gas carried particulate pollutants through counter current regenerative heat exchange. The invention also refers to an equipment for accomplishment of the process.

The rate of a combustion reaction is very sensitive to temperature andthe rate increases strongly with increasing temperature. In a combustionprocess it is necessary that the temperature of the flame or combustionzone is high enough to make the reaction proceed with reasonable speed.

In an ordinary flame this is accomplished by the fact that heat fromreacted gas heats not yet reacted gas. For a flame to survive it is thennecessary that the heat generated in a gas volume during the reaction isenough both to cover losses to the environment and to heat adjacent notyet reacted gas to a temperature sufficiently high to make thecombustion process proceed there.

Even for good fuels like hydrogen and hydrocarbons the case may be thatgenerated heat is not sufficient. The flame then dies. This is the casewhen the concentration of burnables in an air mixture is too low (or sohigh that the oxygen concentration is too low). When judging the dangerfor a possible explosion a common concept is lower (or upper) explosionlimit.

In many cases there are emissions of inherently energy rich and easilycombusted substances where, however, their concentrations are so lowthat they cannot support an ordinary combustion. This is the case forinstance with evacuated air from many paint and printing shops.

To purify such air by combustion of the solvents there are twoconventional ways:

1. The administration of heat to the air for instance by means ofsupport flames so that the whole gas mixture is heated to a temperaturehigh enough. This method is simple and usually works well, but when theoriginal concentration is low it demands much energy for the heating,which makes this method expensive.

2. The use of catalysts. By using catalysts it is possible to make acombustion process proceed at a comparatively low temperature. This isdone by making the gas pass over the catalyst which has the capabilityto make a combustion process proceed although both temperature andconcentrations of burnables are low. Hereby much energy is savedcompared to method 1 above. However, the method has some drawbacks, forinstance:

(a) Catalysts are expensive.

(b) Catalysts are easily destroyed by certain impurities in the gas evenif their concentration is low (catalyst poisons).

(c) Catalysts usually are sensitive to temperature and destroyed at toohigh temperatures. Consequently, gas mixtures with too highconcentrations of burnables cannot be treated and the whole process issensitive to variations of concentration of burnables.

The present invention relates to a process for the establishment ofcombustion and/or decomposition of pollutants in the form of gas orparticles carried by air or other gas.

The main object of this invention is to bring about a process of theabove mentioned kind which makes possible an effective and reliablecombustion and/or decomposition of pollutants for instance obtained inindustry such as air evacuated from paint spraying booths or nitrogenoxides and thereby prevent harmful gases and particles from entering theenvironment.

This object is attained by a process according to the present inventionof which the main characteristics are that the pollutants are fed intoan apparatus in which the pollutants are heated to self combustionand/or decomposition temperature while warmer treated gas flowingthrough the bed in direction towards the exhaust part of the combustiondevice is allowed to heat inflowing gas and/or by air or other gascarried particles by means of regenerative countercurrent heat exchange.

A second object of the invention is to present an apparatus by means ofwhich the above mentioned process can be practised.

Said second object is accomplished by an apparatus according to thepresent invention characterized by said apparatus incorporating a bedfilled with sand, stone or the like having the ability to store heat andexchange heat and having means for heating of the inner portion of saidbed to self decomposition temperature and/or self combustion temperaturefor instance by means of an electric heater in the bed or by means ofgaseous fuel.

The invention is described below as a preferred arrangement wherebyreference is made to the enclosed drawing which schematically shows anapparatus capable of performing the process according to the invention.

A "combustor" 1 which preferably is suited to be used effectivelyaccording to the invention comprises a bed of sand, stone or othermaterial which has the ability to store and exchange heat and to combustand/or decompose gaseous or particulate pollutants carried by air orother gas in the bed. For instance air evacuated from pain sprayingbooths or other gases which contain burnable gases etc. and/or forinstance nitrogen oxides or other gases possible to decompose can becombusted and/or decomposed in said bed 1. This is only due to the hightemperature, i.e. without the bed participating in the reactionotherwise than as a means to establish the high temperature. For thepurpose of accomplishing an effective self combustion and/or selfdecomposition of the pollutants in the bed 1 the latter is equipped withmeans of appropriate kind to heat it preferably in the middle portion ofthe bed to desired self decomposition and/or self combustiontemperature, for instance by means of an electric heater or by means ofoil or gaseous fuel. A conduit 2 communicating with the two ends 1A and1B respectively of the combustor 1 also communicates with a device 3 forperiodic changing of the direction of flow which can be manually and/orautomatically operated. After start of the apparatus 1, i.e. heating thebed to the self combustion and/or self decomposition temperature of themedium intended to be treated, the intention is to preferably let thepollutants in question be fed into it by means of conduit 2 in order todrive the apparatus 1, but mainly to ascertain a complete combustion ofsaid pollutants which may be harmful or odorous like paint gases.

The feeding of pollutants to the device 3 for changing of flow directionis done through a duct 4 from the inlet 5.

Changing of the direction of flow by means of the device 3 by forinstance an incorporated valve 6 which can be switched between twoalternative positions makes the feeding of pollutants coming via theinlet 5 to the combustor in one or the other of the two directions 7 or8 by means of ducts 9 or 10. Said duct 2 also works as outlet for airand/or gas from the combustor 1 to an exhaust duct 11 which leads fromthe flow direction changer 3 to an exhaust 12.

The counter current regenerative heat exchange occurring in the bedmakes it possible to use the heat contents of the treated gas whichleaves through the bed 1' of the combustor 1 to heat the gas and/or airor of other gas carried particulate pollutants which are being fedinwards through said bed 1'. Preferably this is done so that maximumtemperature and combustion and/or decomposition occurs in the centralparts of the bed 1'. This is accomplished by changing of the directionof gas flow through the combustor 1 and its bed 1' at suitable timeintervals by means of the gas flow direction changer 3. Hereby it ismade possible to keep the maximum of the temperature profile in thecentral parts of the combustor bed where said combustion and/ordecomposition is desired to take place.

After start of operation of the combustor at desired self combustionand/or self decomposition temperature by using the above mentionedheating means the combustion and/or decomposition of the pollutants canoften proceed by influence from the heat of the bed 1' without havingany extra energy supplied by the heating means. This is the case whenheat produced by the reaction in the bed is enough to compensate forunavoidable heat losses from for instance incomplete heat exchange inthe bed.

In cases when the pollutants are rich in energy, produced excess heatcould be utilized by extraction at desired temperature level by means ofcooling tubes installed at suitable positions in the bed.

A storing device M which makes possible a safe reception of pollutantswhich are fed from the inlet 5 to the changing device 3 during the timesaid changing device 3 is being switched over for the reversal of theflow direction in the conduit 2 to the combustor 1 is connected to theoutlet duct 11 after the direction changer 3, as seen in the directionof the flow. This is done to prevent pollutants to escape in connectionwith the above described interchange of point of feed and point ofoutlet of gas/air. Preferably said storing device M comprises acomparatively long duct 13 which makes it possible to temporarily storeair/gas of said kind. A circuit 14 which comprises an entrance duct 15which is connected to the exhaust duct 11 before an incorporated valve16 or the like and an outlet 17 which is connected to the inlet duct 4is connected to said long duct 13 or the like in store M incorporatedreception reservoir. One way valves 18 and 19 which make it possible toconvey air and/or gas in the direction of the arrows 20 and 21 on thedrawing to and from the store M, but are designed to automatically shutoff flow in the opposite direction, are incorporated in the inlet andoutlet ducts 15 and 17, respectively. A valve 22 is incorporated in aduct 23 leading for instance outside from the store M and a fan 24 orsome other type of blowing machinery is connected to the store M. Thisfan can preferably be run continuously while the device is operating.Changing of flow is made possible according to the following: Positionsof the valve 6 at the switching device 3 according to the continuousline in the drawing will convey the pollutants in the direction 7 in theconduit 2 into the combustor 1 and convey treated gas to the outlet 12from the combustor 1 in the direction 25. Reversal of the valve 6according to the dashed line on the drawing will convey the pollutantsfrom the entrance 5 to the combustor 1 in the direction 8 and thustreated gas by means of the circuit 9 to the exhaust 12 in the direction26. When opening valve 22 the one way valve 19 is automatically shut sothat possible gas flowing towards exhaust 12 is instead conveyed intothe store M when valve 16 has been shut off and the one way valve 18opened. This is done when changing the direction of gas flow by means ofthe device 3 and the circuits are blown through for a certain time afterthe change of direction to make pollutants not desired to escape insteadgo into the store M. Thereafter the valve 16 is opened whereby one wayvalve 18 is automatically shut off and valve 22 is closed so that oneway valve 19 automatically opens. Air/gas stored in M is then fed to theinlet duct 4 and to the combustor 1 in the above mentioned way by meansof the fan 24 in the direction of arrow 21. The store M thus makespossible an effective storage of harmful air/gas which otherwise couldhave escaped during the changing of direction of flow.

The invention is not limited to the embodiment described above and shownon the drawing, but can be varied within the limits of the claimswithout differing from the scope of the invention.

I claim:
 1. A process for treatment of gas, vapor, or air-borne particulant pollutants comprising the steps of:feeding the pollutant gas into a chamber having a heated bed with heat-accumulating and heat-exchanging properties, to raise the termperature of the pollutant gas to cause combustion or decomposition thereof; and reversing the direction of gas flow in the chamber to cause treated pollutant gas to raise the termperature of the incoming untreated pollutant gas comprising the step of re-routing pollutant gas from the input of the heated chamber to a storage chamber at the time of the changing of direction of gas flow, and from which storage chamber, the pollutant gas is later routed back into the heated chamber.
 2. A process as claimed in claim 1, comprising the step of heating the central portion of the heated bed to a gas decomposition or combustion temperature, at least during the starting point of the process, by means of a heater positioned within the heated bed, whereby pollutant gas heated by the central portion of the bed will heat the exit-adjacent-portion of the bed.
 3. A process as claimed in claim 1, comprising the step of re-routing partially treated gas pollutant from exhaust discharge and into said storage chamber to avoid exhausting of untreated pollutant gas.
 4. A process as claimed in claim 3, comprising the step of forcing untreated pollutant gas stored in said storage chamber back into the heated chamber for processing.
 5. A process as claimed in claim 1, comprising the step of extracting by means of cooling tubes, the surplus heat generated in said heated chamber.
 6. An apparatus for treatment of gas, vapor, or air-borne particulant pollutants comprising:a heated chamber in the form of a bed of sand, stone, or other material having heat-accumulating and heat-exchanging properties. means for heating through an external fuel source, a central portion of said bed to a temperature that will cause combustion or decomposition of the pollutant gas passing thereover; and means for alternating the direction of pollutant gas into the heated chamber for treatment to cause gas flow in opposite, alternate directions.
 7. Apparatus as claimed in claim 6, comprising means for changing the direction of pollutant gas entry and exit from said heated chamber to cause a reverse in direction of gas flow over the heating means.
 8. Apparatus as claimed in claim 6, comprising a storage means for temporarily storing gas during the change of direction procedure to avoid release from the system of uncompletely treated pollutant gas, and means for returning gas stored in the storage means back to the heated chamber for processing.
 9. Apparatus as claimed in claim 6, comprising cooling tubes positioned within said heating bed to enable extraction of excess surplus heat from the system.
 10. An apparatus for treatment of gas, vapor, or air-borne particulant pollutants comprising:a heated chamber in the form of a bed of sand, stone, or other material having heat-accumulating and heat-exchanging properties; means for heating a central portion of said bed to a temperature that will cause combustion or decomposition of the pollutant gas passing thereover; and means for alternating the direction of pollutant gas into the heated chamber for treatment to cause gas flow in opposite, alternate directions.
 11. Apparatus as claimed in claim 10, comprising means for changing the direction of pollutant gas entry and exit from said heated chamber to cause a reverse in direction of gas flow over the heating means.
 12. Apparatus as claimed in claim 10, comprising a storage means for temporarily storing gas during the change of direction procedure to avoid release from the system of uncompletely treated pollutant gas, and means for returning gas stored in the storage means back to the heated chamber for processing.
 13. Apparatus as claimed in claim 10, comprising cooling tubes positioned within said heating bed to enable extraction of excess surplus heat from the system. 